Tacheometers, the generic term for theodolite/distance measuring instrument combinations, are powerful surveying tools capable of measuring the angles and the distances between points. Typically, this is done by leveling the instrument, optically aligning a gimbaled telescopic sight upon a target, measuring the shaft angles of the sight once aligned, and measuring the distance between the instrument and the target. Shaft angles can be measured by graduated vernier techniques or by more advanced digital encoding techniques described in more detail in the section below entitled THEODOLITE. Techniques for referencing the instrument to level are also described therein. Techniques for measuring distances include parallax methods as well as phase shift techniques such as described in more detail in the section below entitled DISTANCE MODULE. The major drawback of the prior art in tacheometers has been that a small, lightweight, and highly accurate tacheometer has heretofore been unobtainable.
The present invention utilizes a sinusoidal signal interpolator technique to encode shaft angles and 2 axis level angles as phase shifted signals. The distance module similarly encodes distance as a phase shifted signal, allowing both angle and distance measurements to be decoded by a shared phase detector and accumulator. A processor and keyboard allow an operator to select desired measurement sequences and calculations, including the determination of angles and distances that are corrected for off-level conditions detected by the 2-axis level sensor, refraction, earth curvature, and a speed of light correction factor selected by the operator.